Democrats on several top House committees launched an investigation Thursday into the massive shakeup in leadership at the Department of Homeland Security due to allegations that President Trump removed top officials for refusing to carry out his desired immigration policies.

House Oversight Committee Chairman Elijah Cummings, D-Md.; Judiciary Committee Chairman Jerrold Nadler, D-N.Y.; and Homeland Security Committee Chair Bennie Thompson sent a letter to the Department of Homeland Security Thursday seeking documents related to recent immigration-related actions by the president and White House aide Stephen Miller "to remove senior leaders throughout DHS who reportedly refused orders to violate the law."

WHITE HOUSE DECLINES CUMMINGS' INVITATION FOR STEPHEN MILLER TO TESTIFY ON TRUMP IMMIGRATION POLICIES

The request comes amid a staffing shake-up at the agency which began earlier this month with the resignation of DHS Secretary Kirstjen Nielsen.

“We are deeply concerned that the firing and forced resignation of these officials puts the security of the American people at risk,” they wrote. “We are also concerned that the President may have removed DHS officials because they refused his demands to violate federal immigration law and judicial orders.”

They added: “Moreover, we are concerned by reports that, even as he has removed the Department’s leadership, the President has sought to empower a White House aide, Stephen Miller, to ‘be in charge of handling all immigration and border affairs.’”

The letter was sent to DHS just hours after White House Counsel Pat Cipollone notified the House Oversight Committee that Miller would decline Cummings’ invitation to testify before the panel, and instead, offered cabinet secretaries and other agency officials to appear instead.

“In accordance with long-standing precedent, we respectfully decline the invitation to make Mr. Miller available for testimony before the Committee,” he wrote. “The precedent for members of the White House staff to decline invitations to testify before congressional committees has been consistently adhered to by administrations of both political parties, and is based on clearly established constitutional doctrines.”

He added: “In light of the Committee’s interest, we welcome the opportunity to discuss the Administration’s immigration policy priorities and note that many Executive Branch officials, including cabinet secretaries and other agency leaders, have testified on multiple occasions regarding the Administration’s efforts to secure the border.”

“Testimony by such Executive Branch officials with responsibilities defined by statute would be a reasonable accommodation to the Committee’s questions and legislative goals,” he wrote.

The White House’s decision comes after President Trump vowed this week to fight “all” subpoenas against the administration.

TRUMP, AGAIN, SAYS ILLEGAL IMMIGRANTS WILL BE 'GIVEN' TO SANCTUARY CITIES, STATES

But the Oversight Committee did not subpoena Miller. Cummings invited Miller to testify on why he believes it is “good policy for the Trump administration to take the actions it has. Cummings noted the administration’s unofficial policy of separating immigrant children from their parents at the border, “transferring asylum seekers to sanctuary cities as a form of illegal retribution” against political adversaries, and “firing top Administration officials who refuse orders to violate the law.”

The invitation from Cummings came after Trump announced that illegal immigrants would be “given” to sanctuary cities, pending official decisions from the Department of Homeland Security. Also, earlier this month, Trump said that the administration has no plans to revive the controversial policy that allowed for family separations at the border.

That announcement came amid a massive shakeup at the Department of Homeland Security. Earlier this month, President Trump announced that Nielsen would be resigning, and replaced with former CBP Commissioner Kevin McAleenan, who would serve as acting secretary of the agency.

Since then, McAleenan appointed John Sanders to serve as acting CBP commissioner; former head of the Transportation Security Administration David Pekoske to be his acting deputy secretary at DHS; and Patricia Cogswell as acting TSA administrator. The White House earlier this month announced that James M. Murray would become head of the U.S. Secret Service—replacing Randolph “Tex” Alles.

Fox News' Kristin Brown contributed to this report.